Light Blocking Structure in Leadframe

ABSTRACT

A semiconductor proximity sensor ( 100 ) has a flat leadframe ( 110 ) with a first ( 110   a ) and a second ( 110   b ) surface, the second surface being solderable; the leadframe includes a first ( 111 ) and a second ( 112 ) pad, a plurality of leads ( 113, 114 ), and fingers ( 115, 118 ) framing the first pad, the fingers spaced from the first pad by a gap ( 116 ) which is filled with a clear molding compound. A light-emitting diode (LED) chip ( 120 ) is assembled on the first pad and encapsulated by a first volume ( 140 ) of the clear compound, the first volume outlined as a first lens ( 141 ). A sensor chip ( 130 ) is assembled on the second pad and encapsulated by a second volume ( 145 ) of the clear compound, the second volume outlined as a second lens ( 146 ). Opaque molding compound ( 150 ) fills the space between the first and second volumes of clear compound, forms shutters ( 151 ) for the first and second lenses, and forms walls rising from the frame of fingers to create an enclosed cavity for the LED. A layer ( 180 ) of solder is on the second leadframe surface of the pads, leads, and fingers.

FIELD OF THE INVENTION

The present invention is related in general to the field ofsemiconductor devices and processes, and more specifically to thestructure and fabrication method of molded proximity sensors withcomplete light blocking features embedded in the leadframe of thedevice.

DESCRIPTION OF RELATED ART

Semiconductor proximity sensors require a light-emitting chip, typicallya light-emitting diode (LED), and a sensor chip. It is well known thatfor light in the infrared wavelength regime, sensor chips are extremelysensitive to background and stray light. It is therefore mandatory forsemiconductor proximity sensors operating in the infrared regime toconstruct the package so that the package allows only light back intothe sensor, which has first been emitted from the LED on the front sideof the sensor and then comes bouncing back from the intended target. Ifany light would leak from the LED through the back side of the package(so-called cross talk), or if it had been reflected from anything elsebut the intended target, it would cause noise. Only a few percent ofleaked light would cause the sensor to fail.

In known technology of semiconductor sensors, plastic proximity sensorsare being built using dual molding processes. One molding step employsoptically clear compound to create the dome-shaped lenses over thesemiconductor devices in two separate cavities, and the other moldingstep employs optically opaque compound to fill in the space between thecavities and optically isolate the chips; the opaque compound can alsobe used to create shutters and light-blocking walls on the top side ofthe package. Two methodologies are being practiced. When clear compoundis used in the first molding step and opaque compound is used in thesecond molding step, the substrate is made of fiberglass or ceramic toblock any light emitted from the LED and arriving from the back side tothe sensor; the molding step with opaque compound can also create tightshutters. This methodology has to accept the relatively high cost offiberglass- or ceramic-made substrates.

On the other hand, when opaque compound is used in the first moldingstep to create the openings of the cavities and clear compound is usedin the second molding step to fill in the openings and create thelenses, the substrate can be provided by relatively low cost metallicleadframes, since the opaque compound fills in the spaces between leadsand pads for blocking stray light. On the other hand, in this processflow the opaque compound is not available to form shutters orlight-blocking walls on the top side of the package.

SUMMARY OF THE INVENTION

Applicants realized that the relentless cost pressure on semiconductorproximity sensors favors the substrate usage of low-cost leadframes overhigher cost ceramics or fiberglass. In addition, they acknowledged thepracticality of using opaque molding compounds for creating low-cost anddesign-specific shutters. As a consequence, the usage of clear compoundfor the first molding step is the preferred methodology.

Applicants found a solution to the problem that the first-used clearmolding compound fills the spaces between leads and pad of leadframesand thus may create light pipes and unacceptable cross talk in theproximity package, when they discovered a methodology combining thedesign of the leadframe to form a cell of opaque compound around the LEDwith the light-blocking capability of a solder barrier between the LEDside of the proximity package and the sensor side.

One embodiment of the invention is a semiconductor proximity sensorhaving a leadframe with a solder layer on its outer surface. Theproximity sensor includes a flat leadframe, which has a first and asecond surface, with the second surface being solderable. The leadframeis designed to have a first and a second pad, a plurality of leads, andfingers which form a frame around the first pad while they are spacedfrom the first pad by a gap. This gap is filled with clear moldingcompound. The proximity sensor has an LED chip assembled on the firstpad and encapsulated by a first volume of the clear compound, whereinthe first volume is outlined as a first lens, and further a sensor chipassembled on the second pad and encapsulated by a second volume of theclear compound, wherein the second volume is outlined as a second lens.Opaque molding compound fills the space between the first and secondvolumes of clear compound, forms shutters for the first and secondlenses, and forms walls rising from the frame of fingers in order tocreate enclosed cavity for the LED. The proximity sensor includes alayer of solder on the second surface of the leadframe pads, leads, andfingers.

Another embodiment of the invention is a method for fabricating aproximity having a leadframe with a light-blocking structure. The methodstarts by providing a flat leadframe with a first and a second surface;the second surface has a metallurgical surface preparation suitable forsoldering. The leadframe includes a first and a second pad, a pluralityof leads, and fingers framing the first pad, whereby the fingers arespaced from the first pad by a gap.

In the next process steps, a light-emitting diode (LED) chip isassembled on the first pad and a sensor chip is assembled on the secondpad. The assembly of these chips include the steps of adhesivelyattaching the chip to the respective pad and wire bonding the chip to arespective lead.

In the next process steps, a clear polymeric compound is molded to fillthe gap between the fingers and the first pad, further to encapsulatethe assembled LED chip by a first volume of compound outlined as a firstlens, and to encapsulate the assembled sensor chip by a second volume ofcompound outlined as a second lens. Then, an opaque polymeric compoundis molded to fill the space between the first and second volumes ofclear compound, further to form shutters for the first and secondlenses, and to form walls rising from the frame of fingers, thuscreating an enclosed cavity for the LED, which can prevent light fromescaping from the cavity.

In the final process step, the proximity sensor is attached to a board(PCB) using a layer of solder to connect to the second leadframesurface. As an example, the solder layer may be provided by screening asolder paste. The solder layer blocks the escape route for light throughthe clear compound filling the gap between the first pad and theencircling leadframe fingers.

Alternatively, the leadframe is already provided with a layer of solderpre-deposited on the second leadframe surface. A preferred solderincludes tin or a tin alloy; alternative solders are indium-gold andother low-melting binary alloys.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross section of a semiconductor proximity sensorassembled on a leadframe; a solder layer is pre-deposited on theleadframe, or is provided during the attachment step to a board.

FIG. 2 shows a top view of the proximity sensor of FIG. 1, depicting aembodiment of the leadframe fingers framing the assembly pad of thelight-emitting diode.

FIG. 3 shows a top view of the proximity sensor o FIG. 1, depictinganother embodiment of the leadframe fingers framing the assembly pad ofthe light-emitting diode.

FIG. 4 illustrates a cross section of another semiconductor proximitysensor assembled on a leadframe; the device having opposite sidesun-covered by opaque compound; a solder layer is pre-deposited on theleadframe, or is provided during the attachment step to a board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an exemplary semiconductor proximity sensor generallydesignated 100, which employs a metallic leadframe 110 with a first (110a) and a second (110 b) surface and a layer 180 of solder on the secondsurface. The components of proximity sensor 100 are assembled on theleadframe surface opposite the solder layer; the design of the leadframeis displayed in the top view of FIG. 2 before the assembly. As anexample, proximity sensor 100 and leadframe 110 may have a length 101 of4.0 mm and a width 102 of 2.0 mm; other proximity sensors may have thedimensions of length 4.0 mm and width 2.5 mm, or length 5.0 mm and width2.5 mm, or length 3.94 mm and width 2.36 mm. Overall height 103 of thedevice may be 1.35 mm.

Proximity sensor 100 includes a light-emitting diode (LED) chip 120 anda sensor chip 130. In the example of FIG. 1, the light emitted by theLED is in the infrared regime; in other embodiments the light may be inthe regime of shorter wavelengths. Chips 120 and 130 are encapsulated involumes 140 and 145, respectively, of clear plastic compound. Thecompound is shaped as lenses 141 and 146, respectively. The plasticvolumes are separated by opaque plastic compound 150, which alsooutlines the overall contours of the device package.

Referring now to FIG. 2, the top view shows the first surface ofleadframe 110; the opposite second surface of leadframe 110 is not shownin FIG. 2. Leadframe 110 includes a first pad 111 for assembling theLED, a second pad 112 for assembling the sensor, a plurality of leads113 (dedicated to the LED) and 114 (dedicated to the sensor), andfingers 115 and 118. In the exemplary embodiment, the leads as packageterminals are not shaped as conventional cantilevered leads, but flatmetal pins; consequently, the embodiment is classified as a plasticSmall Outline No-lead (SON) package, frequently also called a Quad FlatNo-lead (QFN) package.

It should be noted that herein, following widespread usage, packageterminals 113 and 114 are referred to as pins, in spite of the fact thatthey have a flat surface and do not resemble pointed objects such asnails. When a leadframe is used for an embodiment to assemble asemiconductor chip on the leadframe pad and connect the chipinput/output terminals to the leadframe leads, those leads are hereinalso referred to as pins. In the embodiment shown, the metal pins arecoplanar with the surrounding plastic surface; in other embodiments,they may protrude a step of about 0.05 mm from the plastic surface.

The preferred base metal for the leadframe in FIG. 2 is copper or acopper alloy. Base metal alternatives include brass, aluminum,iron-nickel alloys (for instance the so-called Alloy 42), and Kovar™.Typically, the leadframe originates with a metal sheet with a preferredthickness in the range from about 100 to 300 μm; thinner sheets arepossible. If needed, the ductility in this thickness range provides the5 to 15% elongation that facilitates an intended bending and formingoperation. The configuration or structure of the leadframe is stamped oretched from the starting metal sheet.

As defined herein, the starting material of the leadframe is called the“base metal”, indicating the type of metal. Consequently, the term “basemetal” is not to be construed in an electrochemical sense (as inopposition to “noble metal”) or in a structural sense.

FIG. 1 indicates that pads 111 and 112 serve the adhesive attachment andassembly of chip 120 and chip 130, respectively. FIG. 1 also shows thatleads 113 and 114 serve the attachment of wire stitch bonds on the firstsurface of the leadframe. Consequently, the first surface of the leadshas to be bondable. Dependent on the selected base metal of theleadframe, bondability may have to be enhanced by a metallurgicalpreparation of the first surface of the leads, such as additional spotplating with a layer of silver, or palladium, or gold. It is shown inFIG. 1 that the second surface of the leadframe, including pads, leads,and fingers, is suitable for soldering. This requirement may necessitatea metallurgical preparation of the second surface, such as plating thebase metal with a layer of tin, or with a layer of nickel followed by athin layer of palladium, or palladium and gold, and then a thick layerof tin or tin alloy.

For some devices it may be advantageous to half-etch at least portionsof the pads, leads, and fingers in order to create structures, whichenhance molding compound adhesion and thermal heat spreading. Suitablestructures include rims as mold locks.

Referring now to FIG. 2, the leadframe fingers 115 and 118, which framefirst pad 111, are spaced from pad 111 by a gap of width 116. Fingers115 and 118 are designed so that they encircle first pad 111 to form acell onto which a wall of opaque molding compound can be fabricated;this walled-in cell acts a cavity for the LED assembled on pad 111, fromwhich none or only minimal light can escape. As is pointed out below,when the gap of width 116 is filled with clear molding compound and thusstray light could manage to escape through the clear-filled gap, it willbe fully blocked from progressing further by solder layers 180 (see FIG.1).

In the embodiment of FIG. 2, fingers 115 leave an opening 117 forplacing lead 113, onto which the stitch bond of the LED bonding wire 160is attached. In another embodiment shown in FIG. 3, fingers 315 encirclea fully closed cell. Pad 313 for the stitch bond of the LED binding wire360 is re-arranged inside the cell.

Referring to FIG. 1, the LED chip 120 attached onto first pad 111 andwire-bonded to lead 113 is encapsulated by a first volume 140 of clearmolding compound, which is bordered beyond diameter 142 by opaquemolding compound 150. For the exemplary embodiment of FIG. 1, thecharacteristics of “clear” refer primarily to infrared lightwavelengths, and the characteristic of “opaque” refer primarily theinfrared wavelengths. For other embodiments, the characteristics of“clear” may refer to other light wavelengths, especially in the visiblewavelength regime, and “opaque” may refer to blocking primarily thevisible wavelengths. As FIG. 1 indicates, the first compound volume isoutlined by a contour configured as a first lens 141. The contours oflens 141 intend focus the light emitted by the LED within theoperational regime of device 100.

The sensor chip 130 attached onto second pad 112 and wire-bonded toleads 114 is encapsulated by a second volume 145 of clear moldingcompound, which is bordered beyond diameter 147 by opaque moldingcompound 150. Second volume 145 has a contour configured as a secondlens 146. The contour of second lens 146 intends to collect infraredlight arriving as reflected signal and forward the light to sensor 130.In addition, the clear molding compound of second volume 145 has anappended portion 145 a, which encapsulates the full extent of sensorchip 130. Appendix 145 a, in turn, is overlaid by a layer 151 of opaquecompound. As stated above, for the exemplary embodiment of FIG. 1, thecharacteristics of “clear” refer primarily to infrared lightwavelengths, and the characteristic of “opaque” refer primarily theinfrared wavelengths, while for other embodiments, the characteristicsof “clear” may refer to other light wavelengths, especially in thevisible wavelength regime, and “opaque” may refer to blocking primarilythe visible wavelengths.

As FIG. 1 shows, the opaque compound provides a plurality of functions.It fills the space between first volume 140 and second volume 145; itacts as shutters to define the diameters 142 and 147 of the first lens141 and the second lens 146; it covers as appendix 151 the extent oflight sensor 130 beyond diameter 147; and it further forms the walls 104rising from the fingers (115 in FIGS. 1 and 315 in FIG. 3) of theleadframe to create an enclosed cavity of LED chip 120.

As mentioned above, the layers 180 of solder, which attach the secondsurface of all leadframe portions to metallized pads on a PCB surfaceduring the solder reflow step, may be provided by plated layers on thesecond leadframe surface; alternatively, it may be provided by solderpaste screened or deposited on the PCB before the attachment.Independent of the method of providing the solder alloy, the solderlayers under fingers 115 and leadframe portion 118 will block any lightescaping from the cavity surrounding LED chip 120 and thus prevent anynoise disturbing the proper operation of sensor 130.

Another embodiment of the invention is a method for fabricating aproximity sensor 100 having a leadframe with a light-blocking structure.The method starts by providing a flat leadframe 110 with a first and asecond surface; the second surface has a metallurgical surfacepreparation suitable for soldering. The leadframe includes a first and asecond pad (111 and 112, respectively), a plurality of leads (113, 114),and fingers (115, 118) framing the first pad, whereby the fingers arespaced from the first pad by a gap 116.

In the next process steps, a light-emitting diode (LED) chip 120 isassembled on first pad 111 and a sensor chip 130 is assembled on secondpad 112. The assembly of these chips include the steps of adhesivelyattaching the chips to the respective pads and wire bonding the chips torespective leads.

In the next process steps, a clear polymeric compound is molded to fillthe gap 116 between the fingers and the first pad, further toencapsulate the assembled LED chip 120 by a first volume 140 of compoundoutlined as a first lens 141, and to encapsulate the assembled sensorchip 130 by a second volume 145 and 145 a of compound outlined as asecond lens 146. Then, an opaque polymeric compound is molded to fillthe space between the first and second volumes of clear compound,further to form shutters (such as 151) for the first and second lenses,and to form walls rising from the frame of fingers (such as wall 150over leadframe portion 118), thus creating an enclosed cavity for theLED, which can prevent light from escaping from the cavity.

In the final process step, the proximity sensor with its pads, leads,and fingers is attached to a board (PCB) using a layer 180 of solder toconnect to the second leadframe surface. As an example, the solder layermay be provided by screening a solder paste and reflowing it during theattachment step. Solder layer 180 blocks the escape route for lightthrough the clear compound filling the gap 116 between the first pad 111and the encircling leadframe fingers 115 and 118.

Alternatively, the leadframe is already provided with a layer of solderpre-deposited on the second leadframe surface. A preferred solderincludes tin or a tin alloy; alternative solders are indium-gold andother low-melting binary alloys.

For devices where miniaturization of the dimensions is at a premium,FIG. 4 illustrates an exemplary embodiment 400 with the opposite sides405 and 406 of the device un-covered by the opaque molding compound.While device 400 of FIG. 4 is generally analogous to device 100 of FIG.1, device 400 has a reduced length 401 compared to length 101 by notplacing opaque molding compound at the opposite sides of the device andthus leaving these opposite sides without light-absorbing cover. Whilethis feature may allow some light to escape from the LED 120, cross talkto sensor 130 is still minimized since the clear compound 140 and 145 ais only exposed on opposite ends of the package of device 400.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an example, the invention applies not only to SON/QFNpackages with side lengths of 4.0 mm, but to packages with scaleddimensions, especially to packages with smaller side lengths.

As another example, the concept of a small plastic leadframe-basedpackage with SON/QFN pins and clear and opaque encapsulation compoundscan be applied to packages with various lens shapes and lens distancessuitable for light wavelengths other than infrared.

In yet another example, the material and the thickness of the metalleadframe can be selected as a function of the size of the chip so thatspecific product goals of the assembled package can be achieved such asfinal thickness, mechanical strength, minimum warpage, prevention ofcracking, strong symbolization contrast, compatibility withpick-and-place machines, and minimum electrical parasitics. In addition,the starting metal of the plate may be roughened, or plated with metallayers (such as nickel, palladium, gold, and tin), to improve adhesionto polymeric compounds and solderablity to PCBs.

It is therefore intended that the appended claims encompass any suchmodifications or embodiments.

We claim:
 1. A semiconductor proximity sensor comprising: a flatleadframe having a first and a second surface, the second surface beingsolderable, the leadframe including a first and a second pad, aplurality of leads, and fingers framing the first pad, the fingersspaced from the first pad by a gap; clear molding compound filling thegap; a light-emitting diode (LED) chip assembled on the first pad andencapsulated by a first volume of the clear compound, the first volumeoutlined as a first lens; a sensor chip assembled on the second pad andencapsulated by a second volume of the clear compound, the second volumeoutlined as a second lens; opaque molding compound filling the spacebetween the first and second volumes of clear compound, forming shuttersfor the first and second lenses, and forming walls rising from the frameof fingers to create an enclosed cavity for the LED; and a layer ofsolder on the second leadframe surface of the pads, leads, and fingers.2. The proximity sensor of claim 1 wherein the fingers framing the firstpad are designed to leave a gap between the finger tips for placing alead.
 3. The proximity sensor of claim 1 wherein the fingers framing thefirst pad are designed to encircle the first pad as well as the leadcarrying the LED stitch bond.
 4. A method for fabricating asemiconductor proximity sensor comprising the steps of: providing a flatleadframe having a first and a second surface, the second surface beingsolderable, the leadframe including a first and a second pad, aplurality of leads, and fingers framing the first pad, the fingersspaced from the first pad by a gap; assembling a light-emitting diode(LED) chip on the first pad; assembling a sensor chip on the second pad;molding clear compound to fill the gap, encapsulate the assembled LEDchip by a first volume of compound outlined as a first lens, andencapsulate the assembled sensor chip by a second volume of compoundoutlined as a second lens; then molding opaque compound to fill thespace between the first and second volumes of clear compound, formshutters for the first and second lenses, and form walls rising from theframe of fingers for creating an enclosed cavity for the LED; andconnecting the pads, leads, and fingers to a board using a layer ofsolder for attaching the proximity sensor.
 5. A method for fabricating asemiconductor proximity sensor comprising the steps of: providing a flatleadframe having a first and a second surface, the second surface beingsolderable, the leadframe including a first and a second pad, aplurality of leads, and fingers framing the first pad, the fingersspaced from the first pad by a gap; depositing a layer of solder on thesecond surface of the leadframe; assembling a light-emitting diode (LED)chip on the first pad; assembling a sensor chip on the second pad;molding clear compound to fill the gap, encapsulate the assembled LEDchip by a first volume of compound outlined as a first lens, andencapsulate the assembled sensor chip by a second volume of compoundoutlined as a second lens; and then molding opaque compound to fill thespace between the first and second volumes of clear compound, formshutters for the first and second lenses, and form walls rising from theframe of fingers for creating an enclosed cavity for the LED.